Wolff rearrangements, silenes

Wolff rearrangements were also observed when most of the same acylsi-lyldiazoalkanes were photolyzed in acetone instead of benzene.21 The ketenes 185 resulting from a 1,3-methyl migration of the silene were detected in addition to the expected ene product 186 derived from the reaction of the silene with acetone (or other enolizable ketones) (Eq. 58). When R = Ad, only the cyclic siloxatene 187 was formed under the same... 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

When diazoester 198 was subjected to gas-phase pyrolysis and ethanol was added afterwards, a partly different result was obtained (equation 63). AUcoxyacetate 199 is again considered to result from the Wolff rearrangement, but since both ethoxy groups in 200 stem from the added alcohol, a silene cannot be the precursor. In a separate experiment, the doubly rearranged ketene 202 was isolated and shown to react quantitatively with ethanol to give 200. Also interesting is the formation of a small amount (4%) of S-lactone 201, which appears to result from 1,4-C,H insertion of the carbene. No product of an intramolecular C,H insertion could be found after photolysis of ethyl (trimethylsilyl)diazoacetate in an inert solvent ... 

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